With the trend of weight reduction and higher stress of an automobile and the like, a higher stress resistance is required of a valve spring, a clutch spring, and a suspension spring used for an engine, a clutch, and a suspension, thus the stress loaded on a spring tends to increase more and more, and hence a spring excellent in fatigue strength is desired.
In recent years, most of valve springs and suspension springs: use quenched and tempered spring steel wires called oil-tempered wires as the materials; and are produced by being wound into the shape of a spring at ordinary temperature.
Such an oil-tempered wire: has advantages that it can easily obtain a high strength and is excellent in fatigue characteristic and sag resistance since the metallographic structure thereof is a tempered martensite structure; but has the disadvantage that large-scale equipment and treatment cost are required for heat treatment including quenching and tempering. To cope with that, a steel wire of a type that is formed into the shape of a spring by applying cold-winding as it is wiredrawn is known. For example, a piano wire type V is stipulated particularly as a steel wire for a valve spring and a spring conforming to the valve spring among the piano wires in JIS standard (JIS G3522).
In the case of a spring produced not by the application of heat treatment including quenching and tempering as stated above but by cold-drawing (this sort of spring is hereunder referred to as “hard-drawn spring” occasionally), heat treatment is not required and hence the production cost can be reduced. To the contrary, a spring steel wire produced by drawing a steel wire rod having a ferrite/pearlite structure or a pearlite structure without the application of heat treatment has drawbacks such as a low fatigue characteristic and low sag resistance. With such a steel wire rod being used as the raw material, a steel spring having performance satisfying increasingly sophisticated recent requirements is hardly obtained.
With regard to a hard-drawn spring that can be produced at a low cost too, various studies have been done with the aim of obtaining spring performance of a higher level and the present applicants have proposed a technology disclosed in JP-A No. 2002-180200 (Patent document 1). In Patent document 1, a high strength, for example a tensile strength of 1,890 MPa or higher in the case of a wire diameter of 3.5 mm, is obtained and excellent sag resistance is secured by stipulating a pearlite fraction in a hard-drawn spring steel wire in relation to a carbon content and further fining a pearlite nodule size by containing V as an essential element.
With the increase of strength merely by the increase of a carbon content however, the deterioration of wire drawability and toughness cannot be avoided and the increase of the pearlite fraction causes industrial productivity to be limited. Moreover, since the hardenability of a steel increases when V is added, the wire speed has to be reduced at a patenting process required before wiredrawing in order to obtain a pearlite structure and that causes productivity to lower and thus production cost to increase.
Meanwhile, as another technology, the present applicants: have developed a steel wire having improved longitudinal crack resistance by forming pearlite as the main phase and reducing the ferrite area ratio at the surface part as a high-carbon steel material used for the production of a fine wire such as a steel cord or a wire rope; and have disclosed the technology in JP-A No. 2000-355736 (Patent document 2).
The steel wire resembles a steel wire according to the present invention in terms of (1) the main phase is composed of pearlite and thereby the ferrite area ratio is suppressed at the surface part, (2) a B content is stipulated in relation to a Ti content and a N content in order to suppress the generated ferrite amount at the surface part, and (3) not only a total B amount (the same as a B amount in steel) but also a solid solute B amount is controlled.
The technology disclosed in Patent document 2 however is a steel material that is intended to be applied to an ultrafine wire comprising a high-carbon steel wire having a relatively high carbon content such as a steel cord, a bead wire, or a wire rope and is aimed at improving the longitudinal crack resistance required for heavy wiredrawing and the applications and the required characteristics thereof are different from the present invention that is intended to be applied to a spring steel wire used for a valve spring and a suspension spring comprising medium-carbon steel and is aimed at improving a spring fatigue characteristic and wire drawability.
Moreover, the technology focuses only on wiredrawing limit, does not refer to a fatigue characteristic, and is an invention different in kind from the applied present invention on the point that nothing has been pursued from the viewpoints of suppression of the segregation of impurity elements (phosphorus and other elements) by the segregation of free B (solid solute B) to pearlite nodules, the accompanying wire drawability, and improvement of strength and ductility, as those will be described later in detail.
Further, according to the confirmation by the present inventors, the steel wire disclosed in Patent document 2 is a very useful steel grade on the point that the steel wire has a high strength of a 4,000 MPa level in the case of a small diameter but the steel wire has not necessarily been satisfactory as a spring steel wire.